The invention pertains to keyboards and more particularly to keyboards using key switches to control electrical signals.
Key switches can be used for many control and selection functions on devices such as typewriters, telephone handsets and calculators. The keyboards of such devices, since they control electronic signals, try to avoid the use of mechanical make and break contact switches because of their complexity, unreliability and cost. Thus, these keyboards utilize a variety of transducers to perform the switching operation. People have tried to use mechanically operated reed switches, Hall effect devices, photoelectronic devices and capacitive techniques. However, all of these electronic keyboard switches have less mechanical hysteresis than conventional "over-center" key switches.
Mechanical hysteresis is the phenomenom which occurs when a keyboard is depressed and an electrical signal is generated at a first point (make point) during the downward travel of the key. This signal cannot be generated again until the key is released and travels a certain distance to a second point (break point) above a first point and then travels downward again past the first point. The distance between the first and second points is the mechanical hysteresis. However, it is usually expressed as percentage of total key travel. For example, if the spacing between the first and second points is 0.010 inch and the total possible key travel is 0.20 inch then the key is said to have 5% mechanical hysteresis.
Usually, the make and break points are determined by the make and break levels of a sensing amplifier. Generally, a sensing amplifier in the form of a bistate device is utilized. The bistate device has electronic hysteresis so that the device assumes a first state upon receipt of a signal having a first level (the make level) and assumes a second state when the received signal is at a different (lower) level. The voltage difference between the two levels is the electronic or electrical hysteresis of the device. These first and second levels determine the level of the signals required from the key switch. If the level of the signal transferred from the key switch is a monotonic function of key switch displacement, then the first and second levels of the sensing amplifier fix the make and break points of the key switch.
It should be apparent that for a sensing device having a given electrical or electronic hysteresis one would want a key switch transducer which has a great mechanical travel or hysteresis between the generation of the two different signal levels recognized. The reason is that the larger the mechanical hysteresis the smaller is the probability of emitting more than one signal from a single operation of the key switch transducer.
Of the many key switch devices, it has been found that capacitive switch devices (wherein the key acts as a coupling element between two coplanar pads) characteristically have the smallest mechanical hysteresis properties. In spite of this poor mechanical hysteresis property of conventional capacitive switch devices, they afford the greatest economy of manufacture since they rely on the positions of conductive elements that are easily printed on substrates. Therefore, it is highly desirable to utilize capacitive switch devices in multiswitch arrays such as keyboards.